1. Field of the Invention
The present invention relates generally to passive microwave devices and more particularly to microwave power divider/combiners.
2. Description of the Related Art
Microwave devices are generally divided into the broad categories of passive and active devices. Included under the heading of passive microwave devices are microwave hybrids and microwave couplers which are multiport networks that are specifically configured for signal routing between the network ports. A device port into which power is normally fed is typically referred to as an incident port or an input port. A port from which power is extracted is called a coupled port or an output port and other ports (from which power is not extracted) are called isolated ports.
The term coupler usually refers to signal routing devices in which the power at one output port is different from that at another output port. In an exemplary four-port directional coupler, most of the power incident upon port 1 is directed to port 2 with a smaller portion (e.g., -20 dB) directed to port 3 and only a leakage portion reaching port 4. In terms of power ratios P.sub.n /P.sub.m, this coupler is said to have a coupling factor=P.sub.1 /P.sub.3, a directivity=P.sub.3 /P.sub.4 and an isolation=P.sub.1 /P.sub.4. The low-level signal at port 3 is especially useful for monitoring a characteristic of the port 2 signal (e.g., its power level) or for serving as a sample of the port 2 signal in a control process (e.g., as in a feedback control loop).
In contrast to microwave couplers, microwave hybrids generally divide the power at each of a plurality of input ports into equal portions, transmit each of the divided portions to a respective one of a plurality of output ports and combine the transmitted powers at each output port. Accordingly, microwave hybrids are often called power divider/combiners.
An exemplary four-port (2.times.2) power divider/combiner has two input ports and two output ports. In a perfect divider/combiner, the incident power at each input port would be divided into two equal portions which are each transmitted to a respective one of the output ports (i.e., the power division is perfect). None of the incident power would be reflected from the input ports and none of the power at any one of the input ports would be transmitted to the other input ports (i.e., the input ports are perfectly matched to their power sources and the isolation between input ports is perfect).
FIG. 1A illustrates a conventional 2.times.2 divider/combiner 20 which has two input ports 22 (labeled as ports 1 and 2) and two output ports 24 (labeled as ports 3 and 4) that are formed with short transmission-line members 25 having a characteristic impedance Z.sub.c (e.g., 50 ohms). The input ports are coupled with branch transmission members 26 which have a length of .lambda..sub.g /4 (where .lambda..sub.g is the guide (i.e., transmission-line) wavelength of signals at the design center of the divider/combiner). Each input port is coupled to a respective output port with through transmission members 28 which also have a length of .lambda..sub.g /4.
Typically, the branch transmission lines have an impedance Z.sub.c and the through transmission lines have an impedance Z.sub.c /(2).sup.1/2. In FIG. 1A, the divider/combiner 20 is shown as it would appear if its transmission-line members were realized with-planar transmission lines (e.g., microstrip). Because the impedance of planar transmission lines is inversely related to their width, the through transmission lines 28 are wider than the branch transmission lines 26.
FIG. 1B illustrates another 2.times.2 divider/combiner 30 which is similar to the divider/combiner 20 of FIG. 1A with like elements represented by like reference numbers. However the divider/combiner 30 is realized with circular transmission-line members and is typically called a hybrid ring.
The 2.times.2 divider/combiner 20 of FIG. 1 can be used as a building block to construct larger divider/combiners such as the 4.times.4 divider/combiner 40 of FIG. 2 which has been described in various references (e.g., see Kawai, Tadashi, et al., "A Branch-Line-Type Eight-Port Comparator Circuit", 1991 IEEE MTT-S Digest, pp. 869-872). Essentially, the divider/combiner 40 is formed from four 2.times.2 divider/combiners 42, 44, 46 and 48 and has input ports 52 (labeled as ports 1, 2, 3 and 4) and output ports 54 (labeled as ports 5, 6, 7 and 8). The 2.times.2 divider/combiners are arranged so that output signals of some divider/combiners become input signals of other divider/combiners. For example, the divider/combiner 42 (the structure within the curved line 42) has its output ports (ports 3 and 4 in FIG. 1A) coupled respectively to be an input port (port 1 in FIG. 1A) for the divider/combiner 44 (the structure within the curved line 44) and an input port (port 2 in FIG. 1A) for the divider/combiner 46 (the structure within the curved line 46).
Although FIG. 2 shows the 4.times.4 divider/combiner arranged in a planar configuration, many other spatial configurations can be devised. For example, the 4.times.4 divider/combiner 60 of FIG. 3 is formed by folding the divider/combiners 42 and 48 of FIG. 2 in one direction and folding the divider/combiners 44 and 46 of FIG. 2 in an opposite direction. Thus the input ports 52 are positioned in one plane and the output ports 54 are positioned in another parallel plane. In FIG. 3, transmission members are indicated by lines and those having an impedance of Z.sub.c,(2).sup.1/2 are indicated with a heavier line than those having an impedance of Z.sub.c.
Another 4.times.4 divider/combiner 70 is shown in FIG. 4 to have input ports 72 (ports given the odd numbers of 1, 3, 5 and 7) and output ports 74 (ports given the even numbers of 2, 4, 6 and 8) which carry the same port labels as the divider/combiner 60 of FIG. 3. As typically described (e.g., see Ohta, Isao, et al., "A Transmission-Line-Type Eight-Port Hybrid",1992 IEEE MTT-S Digest, pp. 119-122), it has a ring 76 of transmission lines which each have a length of .lambda..sub.g /4 and two cross transmission lines 78 which have a length of .lambda..sub.g /2.
Although most conventional power divider/combiners successfully divide powers received at input ports and combine these divided powers at output ports, they typically include an excessive number of transmission-line members. Their use in microwave circuits, therefore, has a negative effect upon the size and weight of these circuits. This effect is emphasized when the hybrid's transmission-line members are realized in waveguide or coaxial form and the effect is especially costly when such realizations are intended for weight-sensitive applications such as spacecraft.
An exemplary spacecraft antenna array has a beam forming network which includes twenty two coaxial 8.times.8 hybrids (each hybrid is formed with twelve 2.times.2 hybrids that are similar to the hybrid 20 of FIG. 1A). The twenty two hybrids weigh approximately 21 kilograms. Because spacecraft launch costs are currently in the region of $88,000 per kilogram, the twenty two hybrids represent $1,848,000 in launch costs. Power divider/combiners that can be realized with less transmission-line members would obviously provide significant cost savings.